Monitoring leaf senescence at the Arnold Arboretum

Posted by Amanda Gallinat

"As fruits and leaves and the day itself acquire a bright tint just before they fall, so the year near its setting. October is its sunset sky; November the later twilight." -Henry David Thoreau, Autumnal Tints

(This Viburnum carlesii at the Arnold Arboretum includes green, yellow and red leaves on the same individual)

On Saturday afternoon, I took a long, winding walk through Boston’s Arnold Arboretum with Richard Primack and BU undergraduates Christina Lupoli and Hilary Colbeth. It was a warm day, and I abandoned my coat at the car—it felt almost like the beginning of autumn, except that so many of the trees around us were bare. And that is precisely why we were there! For the past two years, the Primack Lab has been monitoring leaf color change and drop of about 1000 woody species spanning across the Arnold Arboretum. We want to know what specifically drives plants to senesce, and how climate change might alter this significant annual process.

(Christina personifies a still-green vine while Richard and Hilary channel one that has fully senesced)

Plants unfold their first leaves in the spring quickly in response to warm temperatures. Spring leaf-out signals the start of the growing season, and triggers increases in carbon and nutrient uptake, evapotranspiration, microbial processes, and even insect activity!

Autumn leaf senescence, on the other hand, marks the end of the growing season in New England. The drivers of autumn senescence, and variation in senescence timing between plant taxa are largely unknown. One unique way to approach this gap in the knowledge is through the extensive collections at the Arnold Arboretum! By monitoring broadly across species throughout the season, in multiple years and in several arboreta around the world, the Primack Lab and our partners aim to determine how phylogeny and geography affect the timing of leaf senescence across species. We know that plants leaf out earlier now than they did in Thoreau’s time, but how leaf senescence has changed remains a mystery.

(This senescing Franklinia sp. turns from green to red)